During the semiconductor fabrication process, a semiconductor wafer undergoes multiple processing steps, such as layering, patterning, etching, doping and heat treating, to form a plurality of substantially identical independent chips or dice, each comprising semiconductor devices and interconnect structures. After fabrication the wafer is cut along scribe lines between adjacent dice. The singulated dice or chips are tested and packaged for use in electronic devices.
Unwanted defects introduced during the fabrication process may cause a chip or a plurality of chips to be rendered non-functional. For example, if a dust speck particle falls upon the surface of the wafer, the particle forms a hard mask and prevents exposure of the underlying regions to subsequent processing steps. These defects typically result in an inoperative die and will be detected during the testing phase. More subtle defects are not detectable during testing but can result in premature failure of the integrated circuit during operation. Such failures can be caused by operator error or a process tool that is operating outside of its specifications.
While defect particles are to be avoided, the effect is typically limited to one die or a small number of adjacent dice. By comparison, operator error and process tool problems generally affect all dice on the wafer. Since the wafers are processed in batches, where a batch size ranges from a single wafer to 25 wafers, all wafers in the batch can be similarly affected.
In an effort to determine the cause of defects and premature device failures, it is considered advantageous to track the wafer through the manufacturing process and develop a record of the processing steps to which the wafer was subjected. Thus each wafer includes a unique identification such as a bar code or other identifying indicia printed or laser engraved on the wafer surface. Bar codes comprise a number of spaced apart parallel lines of varying widths, with data encoded in the line width and the line spacing. As the wafer is processed through fabrication tools, an optical probe scans the wafer with an incident light beam in a direction normal to the bar code lines. The incident beam is modulated by the bar code lines to produce a reflected beam that caries a unique signature of the scanned bar code. The reflected light beam is directed to and detected by an electro-optical detector to form an electronic signal representing the scanned bar code. The code is associated with the wafer and thus serves to track the wafer during the fabrication process steps. In lieu of bar codes, it is also known to use laser inscribed dots for wafer identification.
FIG. 1 illustrates a wafer 10 on which are formed a plurality of dice 12. Although for convenience only four such dice 12 are illustrated in FIG. 1, it is known by those skilled in the art that a considerably greater number of circuit dice (typically between about 100 and 1000) are conventionally formed in the wafer 10. Disposed in one corner of each dice 12 is an identification element 16, such as a bar code or laser inscribed dots as discussed above.
It may also be desired to track individual wafer dice by associating each die with a source wafer, a manufacturing lot, a wafer history and/or a die site identifier (i.e., representing the location of the die on the wafer). If the die fails during testing or in the field, the location information can be useful to track defects or yield problems in a wafer region or to identify the source wafer and wafer lot, and thus the processes to which the wafer was subjected during fabrication. In such cases, it may then be appropriate to advise purchasers of the integrated circuits from the suspected lot that in-field failures are possible based on the failure of one or more dice processed in the same lot.
Although identifying information can be physically added to each die by a bar code or laser inscribed dots as described above, this may be avoided due to the extra processing costs incurred. Another known technique for identifying individual chips or dice on a wafer comprises the use of a one-time programmable device such as a laser fuse. A plurality of such fuses are formed on the die and certain ones of the plurality are opened with a laser beam to form a binary pattern of opened and closed fuses. The combination of opened and closed fuses represents a die signature, which is read by passing current through the assembly of fuses. A sense amplifier receives the output current to detect the logic state of each fuse (a zero for an unblown or closed fuse and a 1 for a blown or open fuse, or vice versa). Disadvantageously, the formation of laser fuses (and bar codes) requires the creation of a process mask and execution of additional process steps to form the fuses or bar codes. Also, these indicia consume chip area that could otherwise be devoted to active devices.
In those instances where individual die are identified according to one of the techniques described above, the die identification information is lost after the chip is packaged, unless the package is also marked. To limit costs, packages are typically marked with only wafer lot identifiers and not with die identifiers. Since a wafer lot comprises a substantial number of dice the lot information can be cost-effectively applied, by silk screening, for example, to all packages containing dice from the same lot. Thus die identification information can be determined only by removing the chip from its package.